1. Field of the Invention
The present invention relates to a light source control apparatus used in an image forming apparatus using an electrophotography process, a control method therefor, a storage medium storing a control program therefor, and the image forming apparatus.
2. Description of the Related Art
There are some light source control apparatuses that are used in image forming apparatuses using an electrophotography process are provided with surface emitting lasers as light sources, in general. Although oscillation threshold current of a surface emitting laser is lower than that of an edge emitting laser, a linear area of a current-light characteristic of the surface emitting laser is smaller than that of the edge emitting laser. An optical output of the surface emitting laser increases to the maximum optical output and decreases as driving current increases. Even when the driving current again becomes lower than the value corresponding to the maximum optical output, the surface emitting laser can oscillate. However, when a light control (APC: Auto Power Control) is performed over both of an area in which an optical output increases with increasing driving current and an area in which an optical output decreases with increasing driving current, the surface emitting laser may oscillate in a multimode. Japanese Laid-Open Patent Publication (Kokai) No. 2001-308449 (JP 2001-308449A) discloses a technique that restricts driving current below the value corresponding to the maximum optical output so that a light control is not performed in an area beyond the maximum light amount.
The technique described in the publication determines the maximum light amount that is the optical output where an increasing ratio of the optical output of the surface emitting laser becomes almost zero after the driving current exceeds oscillation threshold current while increasing the driving current gradually and monitoring the light amount of the surface emitting laser. Then, the driving current when the maximum light amount is acquired is specified as an upper limit. Then, an APC circuit controls the light amount so that the driving current does not exceed the upper limit.
Incidentally, when the current-light characteristic of the surface emitting laser varies due to variation of an environmental temperature, it is necessary to calculate the above-mentioned upper limit each time as mentioned below.
As shown in FIG. 4, the optical output of a surface emitting laser monotonously increases until reaching the maximum light amount as applied current gradually increases from zero. The value of the driving current corresponding to the maximum light amount is expressed as Ipeak. When the driving current increases beyond Ipeak, the optical output of the surface emitting laser decreases in monotone. That is, when the driving current increases after reaching the maximum light amount, the optical output decreases as the driving current increases.
It is assumed that the surface emitting laser (called a light emission point, hereafter) is driven by the driving current Is under the APC and operates at a point S where target light amount is acquired at a temperature A, as shown in FIG. 5A. In this case, the high light emission frequency of the light emission point increases the temperature of the light emission point due to the heating of the light emission point itself, which varies the current-light characteristic. For example, it is assumed that the variation of the temperature to B from A varies the current-light characteristic to a curve shown by the temperature B from a curve shown by the temperature A. In this time, when the light emission point is driven by the driving current Is, the operation point of the light emission point shifts to a point S′ as shown in FIG. 5B.
When the operation point of the light emission point shifts to the point S′, the light amount of the light emission point decreases. When the light amount of the light emission point decreases, a CPU will recognize the insufficient light amount in the following APC loop. Accordingly, an APC circuit increases the driving current applied to the light emission point so that the light amount of the light emission point reaches the target light amount based on an instruction from the CPU.
However, the light amount does not reach the target light amount even if the driving current exceeds Ipeak corresponding to the peak light amount, and the APC circuit further increases the driving current (see FIG. 5B). Then, even if the temperature of the light emission point returns from the temperature B to the temperature A immediately, the APC circuit controls to increase the driving current in order to raise the light amount (see FIG. 5C). As a result, although the light amount would reach the target light amount at a point Q when being driven by the driving current Iq, the light amount does not reach the target light amount because the APC circuit controls to increase the driving current.
When the light emission point is operating in the downtrend region, the light amount does not reach the target light amount by increasing the driving current. This does not converge the APC and stops the image formation. Oversupply of the driving current to the emission point in the downtrend region may break the light emission point.
Thus, when the characteristic of the surface emitting laser varies with a temperature variation, the electric current value corresponding to the peak light amount also varies. Accordingly, whenever the characteristics of the surface emitting laser varies, the upper limit of the driving current must be calculated as mentioned above, and the image formation takes long time due to such calculations.